Typically, a vehicle occupant protection apparatus includes a sensor module having an acceleration sensor for outputting an acceleration signal corresponding to acceleration caused by a collision. The occupant protection apparatus detects collision based on the acceleration signal.
In such an occupant protection apparatus, fault diagnosis of the sensor module is performed to ensure proper operation of the apparatus. For example, in an occupant protection apparatus disclosed in JP 2003-2157A, a control section outputs a diagnosis start signal to a sensor module and fault diagnosis of the sensor module is performed in response to the start signal. Then, the sensor module returns a result signal indicating the result of the diagnosis to the control section. The control section determines based on the result signal whether the sensor module is at fault.
The control section cannot distinguish between the acceleration signal and the return signal, because each signal is the same type of signal. Specifically, each of the acceleration signal and the return signal is a voltage signal outputted from the acceleration sensor. Therefore, the control section interprets the first signal, which is inputted to the control section immediately after the control section outputs the start signal to the sensor module, as the result signal.
However, there are fears that the control section misinterprets the return signal as the acceleration signal. Therefore, the control section may activate an occupant protection device (e.g., airbag) despite the fact that the collision does not occur.